mtd: nand: remove the rtc_from4 driver support

The AG-AND support is about to be removed from MTD, because this technology is
dead for long time. Thus, remove this the only AG-AND driver we have in the
kernel tree.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>